1. Field of the Invention
The present invention relates to an optical fiber that can suppress generation of stimulated Brillouin scattering.
2. Description of the Related Art
To realize a large-capacity optical communication, a wavelength-division-multiplexing (WDM) system or a time-division-multiplexing (TDM) system has been adopted. In this kind of communication system, when the optical intensity input to an optical fiber, which is a transmission line, increases, generation of a nonlinear optical phenomenon in the optical fiber becomes prominent. The stimulated Brillouin scattering (SBS), which is one of the nonlinear optical phenomena, is a phenomenon in which a part of a light input to an optical fiber is back scattered, and the scattered light, i.e., Brillouin scattered light causes a stimulated scattering, which is generated by an interaction between a light propagating through the optical fiber and an acoustic wave. When the SBS is generated, it becomes an obstacle to a light propagation in the optical fiber. Since the SBS is generated when the intensity of an input light becomes equal to or larger than a threshold (SBS threshold), it is preferable that the optical fiber used in the transmission line should have a high SBS threshold. A gain experienced when the Brillouin scattered light causes the stimulated scattering is referred to as a Brillouin gain.
So far, as a method of increasing the SBS threshold, methods of changing characteristics of the optical fiber, such as a wavelength dispersion and a transmission loss, in the longitudinal direction by changing a core diameter or a doping amount of a dopant to be doped into the core in the longitudinal direction of the fiber have been proposed (see, for example, Japanese Patent No. 2584151, Japanese Patent No. 2753426, and Japanese Patent No. 3580640). According to these methods, because a shift amount of the Brillouin scattered light with respect to the input light on a frequency spectrum of the light (Brillouin shift amount) is changed in the longitudinal direction of the optical fiber, the SBS can be hardly generated, increasing the SBS threshold. Meanwhile, an optical fiber has been disclosed (see, for example, Japanese Patent Application Laid-Open No. 2006-133314), which can increase the SBS threshold without changing the characteristics of the optical fiber in the longitudinal direction by doping germanium and fluorine into the optical fiber with a portion of the highest fluorine concentration arranged on outside of a portion of the highest germanium concentration.
However, with the optical fibers described in Japanese Patent No. 2584151, Japanese Patent No. 2753426, and Japanese Patent No. 3580640, in which the characteristics are changed in the longitudinal direction, even though the generation of the SBS can be suppressed owing to an increase of the SBS threshold, there is a problem that the characteristics cannot be maintained stable in the longitudinal direction of the optical fiber. As a result, when an optical signal propagates through the optical fiber, there is a possibility of degradation of the optical signal, such as distortion of a signal waveform, when the optical signal passes through a portion of the fiber where the characteristics are locally changed. Although the optical fiber described in Japanese Patent Application Laid-Open No. 2006-133314 does not have a change of the characteristics in the longitudinal direction, the effect of increasing the SBS threshold is not enough.